1. Field of the Invention
The present invention relates generally to a method and apparatus for performing a coronary artery bypass procedure. More particularly, the present invention performs a coronary artery bypass by providing a direct flow path from a heart chamber to the coronary artery. The present invention is suitable for a number of approaches including an open-chest approach (with and without cardiopulmonary bypass), a closed-chest approach under direct viewing and/or indirect thoracoscopic viewing (with and without cardiopulmonary bypass), and an internal approach through catheterization of the heart and a coronary arterial vasculature without direct or indirect viewing (with and without cardiopulmonary bypass).
2. Description of the Prior Art
A. Coronary Artery Disease
Coronary artery disease is the leading cause of premature death in industrialized societies. The mortality statistics tell only a portion of the story. Many who survive face prolonged suffering and disability.
Arteriosclerosis is xe2x80x9ca group of diseases characterized by thickening and loss of elasticity of arterial walls.xe2x80x9d DORLAND""s ILLUSTRATED MEDICAL DICTIONARY 137 (27th ed. 1988). Arteriosclerosis xe2x80x9ccomprises three distinct forms: atherosclerosis, Monckeberg""s arteriosclerosis, and arteriolosclerosis.xe2x80x9d Id.
Coronary artery disease has been treated by a number of means. Early in this century, the treatment for arteriosclerotic heart disease was largely limited to medical measures of symptomatic control. Evolving methods of diagnosis, coupled with improving techniques of post-operative support, now allow the precise localization of the blocked site or sites and either their surgical re-opening or bypass.
B. Angioplasty
The re-opening of the stenosed or occluded site can be accomplished by several techniques. Angioplasty, the expansion of areas of narrowing of a blood vessel, is most often accomplished by the intravascular introduction of a balloon-equipped catheter. Inflation of the balloon causes mechanical compression of the arteriosclerotic plaque against the vessel wall.
Alternative intravascular procedures to relieve vessel occlusion include atherectomy, which results in the physical desolution of plaque by a catheter equipped with a removal tool (e.g., a cutting blade or high-speed rotating tip). Any of these techniques may or may not be followed by the placement of a mechanical support (i.e., a stent) which physically holds open the artery.
Angioplasty, and the other above-described techniques (although less invasive than coronary artery bypass grafting) are fraught with a correspondingly greater failure rate due to intimal proliferation. Contemporary reports suggest re-stenosis is realized in as many as 25 to 55 percent of cases within 6 months of successful angioplasty. See Bojan Cercek et al., 68 AM. J. CARDIOL. 24C-33C (Nov. 4, 1991). It is presently believed stenting can reduce the re-stenosis rate.
A variety of approaches to delay or prevent re-blockage have evolved. One is to stent the site at the time of balloon angioplasty. Another is pyroplasty, where the balloon itself is heated during inflation. As these alternative techniques are relatively recent innovations, it is too early to tell just how successful they will be in the long term. However, because re-blockage necessitates the performance of another procedure, there has been renewed interest in the clearly longer-lasting bypass operations.
C. Coronary Artery Bypass Grafting
i. Outline of Procedure
The traditional open-chest procedure for coronary artery bypass grafting requires an incision of the skin anteriorly from nearly the neck to the navel, the sawing of the sternum in half longitudinally, and the spreading of the ribcage with a mechanical device to afford prolonged exposure of the heart cavity. If the heart chamber or a vessel is opened, a heart-lung, or cardiopulmonary bypass, procedure is usually necessary.
Depending upon the degree and number of coronary vessel occlusions, a single, double, triple, or even greater number of bypass procedures may be necessary. Often each bypass is accomplished by the surgical formation of a separate conduit from the aorta to the stenosed or obstructed coronary artery at a location distal to the diseased site.
ii. Limited Number of Available Grafts
The major obstacles to coronary artery bypass grafting include both the limited number of vessels that are available to serve as conduits and the skill required to effect complicated multiple vessel repair. Potential conduits include the two saphenous veins of the lower extremities, the two internal thoracic (mammary) arteries under the sternum, and the single gastroepiploic artery in the upper abdomen.
Newer procedures using a single vessel to bypass multiple sites have evolved. This technique has its own inherent hazards. When a single vessel is used to perform multiple bypasses, physical stress (e.g., torsion) on the conduit vessel can result. Such torsion is particularly detrimental when this vessel is an artery. Unfortunately, attempts at using artificial vessels or vessels from other species (xenografts), or other non-related humans (homografts) have been largely unsuccessful. See LUDWIG K. VON SEGESSER, ARTERIAL GRAFTING FOR MYOCARDIAL REVASCULARIZATION: INDICATIONS, SURGICAL TECHNIQUES AND RESULTS 38-39 (1990)
While experimental procedures transplanting alternative vessels continue to be performed, in general clinical practice, there are five vessels available to use in this procedure over the life of a particular patient. Once these vessels have been sacrificed or affected by disease, there is little or nothing that modern medicine can offer. It is unquestionable that new methods, not limited by the availability of such conduit vessels, are needed.
iii. Trauma of Open Chest Surgery
In the past, the normal contractions of the heart have usually been stopped during suturing of the bypass vasculature. This can be accomplished by either electrical stimulation which induces ventricular fibrillation, or through the use of certain solutions, called cardioplegia, which chemically alter the electrolyte milieu surrounding cardiac muscles and arrest heart activity.
Stoppage of the heart enhances visualization of the coronary vessels and eliminates movement of the heart while removing the need for blood flow through the coronary arteries during the procedure. This provides the surgeon with a xe2x80x9cdry fieldxe2x80x9d in which to operate and create a functional anastomosis.
After the coronary artery bypass procedure is completed, cardioplegia is reversed, and the heart electrically stimulated if necessary. As the heart resumes the systemic pumping of blood, the cardiopulmonary bypass is gradually withdrawn. The separated sternal sections are then re-joined, and the overlying skin and saphenous donor site or sites (if opened) are sutured closed.
The above-described procedure is highly traumatic. Immediate post-operative complications include infection, bleeding, renal failure, pulmonary edema and cardiac failure. The patient must remain intubated and under intensive post-operative care. Narcotic analgesia is necessary to alleviate the pain and discomfort.
iv. Post-operative Complications
Once the immediate post-surgical period has passed, the most troubling complication is bypass vessel re-occlusion. This has been a particular problem with bypass grafting of the left anterior descending coronary artery when the saphenous vein is employed.
Grafting with the internal thoracic (internal mammary) artery results in a long-term patency rate superior to saphenous vein grafts. This is particularly the case when the left anterior descending coronary artery is bypassed. Despite this finding, some cardiothoracic surgeons continue to utilize the saphenous vein because the internal thoracic artery is smaller in diameter and more fragile to manipulation. This makes the bypass more complex, time-consuming, and technically difficult. Additionally, there are physiological characteristics of an artery (such as a tendency to constrict) which increase the risk of irreversible damage to the heart during the immediate period of post-surgical recovery.
Once the patient leaves the hospital, it may take an additional five to ten weeks to recover completely. There is a prolonged period during which trauma to the sternum (such as that caused by an automobile accident) can be especially dangerous. The risk becomes even greater when the internal thoracic artery or arteries, which are principle suppliers of blood to the sternum, have been ligated and employed as bypass vessels.
v. Less Invasive Procedures
Due to the invasive nature of the above technique, methods have been devised which employ contemporary thoracoscopic devices and specially-designed surgical tools to allow coronary artery bypass grafting by closed-chest techniques. While less invasive, all but the most recent closed-chest techniques still require cardiopulmonary bypass, and rely on direct viewing by the surgeon during vascular anastomoses.
These methods require a very high level of surgical skill together with extensive training. In such situations, the suturing of the bypassing vessel to the coronary artery is performed through a space created in the low anterior chest wall by excising the cartilaginous portion of the left fourth rib. Also, as they continue to rely on the use of the patient""s vessels as bypass conduits, the procedures remain limited as to the number of bypasses which can be performed. Because of these issues, these methods are not yet widely available.
vi. Objectives for Improved Bypass Procedures
In view of the above, it is desirable to provide other methods by which adequate blood flow to the heart can be re-established and which do not rely on the transposition of a patient""s own arteries or veins. Preferably, such methods will result in minimal tissue injury.
While the attainment of the foregoing objectives through an open chest procedure would, by themselves, be a significant advance, it is also desirable if such methods would also be susceptible to surgical procedures which do not require opening of the chest by surgical incision of the overlying skin and the division of the sternum. Such methods would not require surgical removal of cartilage associated with the left fourth rib, would not require the surgical transection of one or both internal thoracic arteries, would not require the surgical incision of the skin overlying one or both lower extremities, and would not require the surgical transection and removal of one or both saphenous veins. In both an open and closed chest approach, it is also be desirable if such methods could be performed without stoppage of the heart and without cardiopulmonary bypass. However, attainment of the foregoing objectives in a procedure requiring cardiopulmonary bypass would still be a significant advance in the art.
vii. References for Prior Art Techniques
The conventional surgical procedures (such as those described above) for coronary artery bypass grafting using saphenous vein or internal thoracic artery via an open-chest approach have been described and illustrated in detail. See generally Stuart W. Jamieson, Aortocoronary Saphenous Vein Bypass Grafting, in ROB and SMITH""s OPERATIVE SURGERY: CARDIAC SURGERY, 454-470 (Stuart W. Jamieson and Norman E. Shumway eds., 4th ed. 1986); LUDWIG K. VON SEGESSER, ARTERIAL GRAFTING FOR MYOCARDIAL REVASCULARIZATION: INDICATIONS, SURGICAL TECHNIQUES AND RESULTS 48-80 (1990). Conventional cardiopulmonary bypass techniques are outlined in Mark W. Connolly and Robert A. Guyton, Cardiopulmonary Bypass Techniques, in HURST""StHE HEART 2443-450 (Robert C. Schlant and R. Wayne Alexander eds., 8th ed. 1994). Coronary artery bypass grafting utilizing open-chest techniques but without cardiopulmonary bypass is described in Enio Buffolo et al., Coronary Artery Bypass Grafting Without Cardiopulmonary Bypass, 61 ANN. THORAC. Surg. 63-66 (1996).
Some less conventional techniques (such as those described above) are performed by only a limited number of appropriately skilled practitioners. Recently developed techniques by which to perform a coronary artery bypass graft utilizing thoracoscopy and minimally-invasive surgery, but with cardiopulmonary bypass, are described and illustrated in Sterman et al., U.S. Pat. No. 5,452,733 (1995). An even more recent coronary artery bypass procedure employing thoracoscopy and minimally-invasive surgery, but without cardiopulmonary bypass, is described and illustrated by Tea E. Acuff et al., Minimally Invasive Coronary Artery Bypass Grafting, 61 ANN. THORAC. SURG. 135-37 (1996).
D. Bypass With Direct Flow From Left Ventricle
1. Summary of Procedures
Certain methods have been proposed to provide a direct blood flow path from the left ventricle directly through the heart wall to the coronary artery. These are described in U.S. Pat. No. 5,429,144 dated Jul. 4, 1995; U.S. Pat. No. 5,287,861 dated Feb. 22, 1994; and U.S. Pat. No. 5,409,019 dated Apr. 25, 1995 (all to Wilk). All of these techniques include providing a stent in the heart wall to define a direct flow path from the left ventricle of the heart to the coronary artery.
As taught in each of the above-referenced patents, the stent is closed during either systole or diastole to block return flow of blood from the coronary artery during the heart""s cycle. For example, the ""861 patent teaches a stent which collapses to a closed state in response to heart muscle contraction during systole. The ""019 patent (particularly FIGS. 7A and 7B) teaches a rigid stent (i.e., open during systole) with a one-way valve which closes during diastole to block return flow of blood from the coronary artery.
ii. Problems
The interruption of blood flow during either diastole or systole is undesirable since such interruption can result in areas of stagnant or turbulent blood flow. Such areas of stagnation can result in clot formation which can result in occlusion or thrombi breaking lose. Such thrombi can be carried to the coronary arteries causing one or more areas of cardiac muscle ischemia (myocardial infarction) which can be fatal. Further, the teachings of the aforementioned patents direct blood flow with a substantial velocity vector orthogonal to the axis of the coronary artery. Such flow can damage the wall of the coronary artery.
Providing direct blood flow from the left ventricle of the coronary artery has been criticized. For example, Munro et al., The Possibility of Myocardial Revascularization By Creation of a Left Ventriculocoronary Artery Fistula, 58 Jour. Thoracic and Cardiovascular Surgery, 25-32 (1969) shows such a flow path in FIG. 1. Noting a fall in coronary artery flow and other adverse consequences, the authors concluded xe2x80x9cthat operations designed to revascularize the myocardium direct from the cavity of the left ventricle make the myocardium ischemic and are unlikely to succeed.xe2x80x9d Id at 31.
Notwithstanding the foregoing problems and scholarly criticism, and as will be more fully described, the present invention is directed to an apparatus and method for providing a direct blood flow path from a heart chamber to a coronary artery downstream of an obstruction. Counter to the teachings of the prior art, the present invention provides substantial net blood flow to the coronary artery.
E. Additional Techniques
Methods of catheterization of the coronary vasculature, techniques utilized in the performance of angioplasty and atherectomy, and the variety of stents in current clinical use have been summarized. See generally Bruce F. Waller and Cass A. Pinkerton, The Pathology of Interventional Coronary Artery Techniques and Devices, in 1 TOPOL""S TEXTBOOK OF INTERVENTIONAL CARDIOLOGY 449-476 (Eric J. Topol ed., 2nd ed. 1994); see also David W. M. Muller and Eric J. Topol, Overview of Coronary Athrectomy, in 1 TOPOL""S TEXTBOOK OF INTERVENTIONAL CARDIOLOGY at 678-684; see also Ulrich Sigwart, An Overview of Intravascular Stents: Old and New, in 2 TOPOL""S TEXTBOOK OF INTERVENTIONAL CARDIOLOGY at 803-815.
Direct laser canalization of cardiac musculature (as opposed to canalization of coronary artery feeding the cardiac musculature) is described in Peter Whittaker et al., Transmural Channels Can Protect Ischemic Tissue: Assessment of Long-term Myocardial Response to Laser-and Needle-Made Channels, 94(1) CIRCULATION 143-152 (Jan. 1, 1996). Massimo et al., Myocardial Revascularization By a New Method of Carrying Blood Directly From The Left Ventricular Cavity Into The Coronary Circulation, 34 Jour. Thoracic Surgery 257-264 (1957) describes a T-shaped tube placed within the ventricular wall and protruding into the cavity of the left ventricle. Also, Vineberg et al., Treatment of Acute Myocardial Infarction By Endocardial Resection, 57 Surgery 832-835 (1965) teaches forming a large opening between the left ventricular lumen and the sponge-like network of vessels lying within the myocardium.
The present invention relates to a method for revascularizing a coronary vessel with a conduit through the heart wall having a diameter transition in the myocardial leg, wherein blood flow is in the direction of transition from larger to smaller diameter. The present invention further relates to revascularizing a coronary vessel using an implant with a transmyocardial leg having a maximum cross-sectional area proximate a first end, and inserting the first end through the myocardium into a heart chamber so that the implant directs blood flow into the coronary vessel. The present invention also relates to a transmyocardial implant with a myocardial leg including point of minimum diameter and a first end with a larger diameter, and a vessel leg in fluid communication with the myocardial leg.